1. Field of the Invention
The present invention relates generally to image processing devices for transmitting image signals for forming hard copy images, and more particularly to an image processing device capable of preventing degradations in the quality of an image resulting from tone reproduction.
2. Description of the Related Art
In an image forming apparatus of a digital processing system such as a digital copier, a facsimile device, or a hard copy system apparatus such as the combination of a computer or a word processor with various types of printers, image information for each pixel of subdivided original images are quantized and handled as digital image data. In these image forming apparatuses, a formation of a binary image is carried out, which is the reproduction of an original image by an arrangement of dots formed of display dots and non-display dots.
When half tone is reproduced in such a binary image, in an image processing device incorporated in the image forming apparatus, image data is binarized compared with threshold data in accordance with an area gradation method represented by a density pattern method or a dither method, and tone reproduction is carried out determining the number of display dots per unit area, depending on the density of the original image.
When the tone reproduction is performed, however, in a dot image portion of the original image such as a printed image of photography, periodical stripe patterns called moire fringe emerge due to a beat phenomenon relating to a repeating period and a dot period of a threshold matrix employed for binarization. Meanwhile, in edge portion of a character image and a line image, display dots become sparse to cause edge loss, i.e., the state that the image is fractionized. Thus, there occurs a degradation in the quality of the images to be formed in both cases.
Therefore, it is necessary for image data to be corrected in advance at the preceding stage of tone reproduction in order to prevent such a degradation in the quality of images.
For preventing the occurrence of moire fringe, smoothing processing known as anti-noise processing is effective, that is, the processing of averaging in weight image data of plural pixels in local areas. For edge loss, the edge emphasizing processing is effective employing a quadratic differential filter (Laplacian filter), which processing is called non-sharp masking for achieving sharpness of an image.
Therefore, as disclosed in Japanese Patent Laying-Open No. 61-157165, for example, the image processing device has conventionally included edge detecting means for controlling the mixing ratio of an output of edge emphasizing means to that of smoothing means as well as the edge emphasizing means and smoothing means, thereby removing the edge loss or moire fringe and also smoothly connecting the character image portion and the dot image portion.
In the binary image formed of definite-size dots, tone and resolution of the image are counter-related to each other. That is, as the number of tones becomes increased, the size of a threshold matrix (unit area for tone reproduction) becomes increased and hence the number of corresponding pixels becomes increased, resulting in a degradation in resolution.
Therefore, the threshold matrix is normally changed depending on the original image for carrying out the tone reproduction. That is, when the original image is mainly a half tone-rich image such as a photograph, the size of the threshold matrix is increased so that the number of tones has priority over resolution. On the other hand, when the original image is a line image such as characters, the size of the threshold matrix is reduced so that resolution has priority over the number of tones.
Further, when a color image is formed by superposition of color toning materials with plural colors, a separate threshold matrix is sometimes provided for each color so that a screen angle (the angle between segments of line passing through the center of each of the threshold matrixes and the direction of main scanning) is different for each color in order to prevent the moire fringe due to a deviation in superposition of the colors.
In the case of tone reproduction by appropriately selecting the threshold matrix as described above, a threshold arrangement is optimized for each matrix, so that the state of the occurrence of edge loss varies for each threshold matrix to be used.
In the conventional image processing device, however, directional property of the quadratic differential filter for edge emphasis is fixed, i.e., the direction of arrangement of a subject pixel and its peripheral pixels affecting the subject pixel. Thus, there has been a problem that changing the threshold matrixes to make them adaptable to the original image causes unrequired emphasizing of edges as well as increased edge loss in a particular threshold matrix, thereby deteriorating reproducibility of an image.
There also has been another problem that eliminating edge loss despite the alteration of the threshold matrixes limits arrangement of the threshold matrix, thereby carrying out no optimal tone reproduction.